1. Field of the Invention
The present invention relates to a reference voltage generating circuit for a liquid crystal display, and more particularly to a reference voltage generating circuit which generates reference voltages for video signals to be provided to a liquid crystal panel.
2. Description of the Prior Art
The conventional liquid crystal display comprises a liquid crystal panel section, a gate driver section, a source driver section, a timing control section, and a fixed reference voltage generating section. Particularly, a liquid crystal display having a liquid crystal panel section, a gate driver section, and a source driver section mounted on the same substrate is called ‘chip-on-glass type liquid crystal display’.
In a liquid crystal panel, pixels, each of which has RGB liquid crystal, are arranged in a matrix pattern, gate lines for driving the pixels are arranged in the row direction and are connected respectively to the gates of transistors in the pixels, and data lines for applying video signals to the pixels are arranged in the column direction and are connected respectively to the sources of transistors in the pixels.
The gate driver section outputs gate signals through the gate lines for each field in response to a gate line control signal.
The source driver section receives signals gamma-corrected on the basis of voltage-transmittance (V-T) characteristics from the timing control section in response to signals of data lines according to the gate signals of the gate driver section, and applies fixed reference voltages selected by RGB data to respective liquid crystal cells.
The timing control section applies RGB data provided from outside to the source driver section, and simultaneously generates a horizontal scanning pulse, a vertical scanning pulse, a polarity reversal pulse POL, a clock pulse CLK, a chip select pulse CS, a shift clock SCLK, a latch signal LT, serial data RSCL and RSDA on the basis of a horizontal synchronizing signal and a vertical synchronizing signal provided from outside, thereby providing the generated signals to the source driver section.
The fixed reference voltage generating means comprises a fixed reference voltage distribution section, a buffer amplification section, and a multiplexer section. The fixed reference voltage generating means outputs reference voltages, which are required when signals having voltages corresponding to RGB digital data are outputted to respective signal lines from data signals of the source driver section, to a source-driver integrated circuit (IC) through the fixed reference voltage distribution section.
FIG. 1 is a circuit diagram for explaining a conventional fixed reference voltage generating means 100.
As shown in FIG. 1, a fixed reference voltage generating means 100 comprises a voltage division circuit 110 including a plurality of resistors R0 to Rn, which are connected in series to each other and located sequentially between two of nodes including reference voltage nodes V1 to Vn and a ground node. The fixed reference voltage generating means 100 receives a source voltage AVDD, and transmits divided voltages V1 to Vn to a multiplexer section (not shown) through a buffer amplification section 120.
The buffer amplification section 120 amplifies the voltages V1 to Vn provided through the resistors R0 to Rn, and transmits the amplified voltages to the multiplexer section. That is, the fixed reference voltage generating means is used to provide instructions indicating a voltage which should be selected from among the reference voltages Vref1 to Vrefn in the source drive IC. Herein, the respective resistors R0 to Rn have the same resistance value with each other. Also, the buffer amplification section 120 uniformly amplifies the reference voltages Vref1 to Vrefn for gamma correction and provides the amplified reference voltages to the multiplexer section.
FIG. 2 is an internal block diagram of a source driver IC for explaining a process in which the reference voltage Vref1 to Vrefn generated in the fixed reference voltage generating means 100 are transmitted to each of data lines.
As shown in FIG. 2, respective reference voltages Vref1 to Vrefn are transmitted to a multiplexer section 200 included in a source driver IC. The multiplexer section 200 classifies the reference voltages Vref1 to Vrefn into changed sets (m1, m2, . . . ) of red reference voltages, green reference voltages, and blue reference voltages, on the basis of polarity reversal pulses, which are alternating currents and used to drive a liquid crystal panel, and transmits the classified reference voltages to a digital-analog conversion section 210. When RGB digital data D0 to Dn supplied from a timing control section (not shown) are level-shifted and transmitted to the digital-analog conversion section 210, the digital-analog conversion section 210 gamma-corrects the digital data D0 to Dn on the basis of the reference voltages Vref1 to Vrefn transmitted from the multiplexer section 200, and applies output signals O1 to On to data lines through a buffer amplification section 220, and so that the output signals O1 to On are transmitted to respective liquid crystals.
For example, in a case in which RGB digital data supplied from outside are 8-bit data (R0˜R7, G0˜G7, B0˜B7), the multiplexer section 200 receives 256 reference voltages for each of RGB signals from the fixed reference voltage generating means 100, selects one of 256 reference voltages Vref1 to Vrefn (V1˜V256) on the basis of the RGB digital data D0 to D7, gamma-corrects the RGB digital data D0 to D7 according to one of red reference voltages, green reference voltages, and blue reference voltages, and transmits the gamma-corrected data to the digital-analog conversion section 210. The digital-analog conversion section 210 converts corrected reference voltages into analog blue signals VBn, analog green signals VGn and analog red signals VRn, and transmits the converted signals to a buffer amplification section 220, and then output signals O1 to On corresponding to a liquid crystal panel are applied to each data line.
Hereinafter, a method for determining the reference voltage values Vref1 to Vrefn will be described as follows with reference to FIG. 3.
FIG. 3 is a graph showing the correspondence relationships between voltage and transmittance. In general, according to screen display principle of a liquid crystal display, when voltages corresponding to respective video information are applied to liquid crystal interposed between pixel electrodes, difference of the applied voltage values causes difference of molecular orientation of liquid crystal, so as to cause difference of transmittance of light, thereby changing color level. At this time, reference voltages VA to VD, which have fixed voltage values determined by the fixed reference voltage generating means, are used.
As shown in FIG. 3, reference voltages VA to VD in the horizontal direction show a particular curve in which transmittance (T) is changed proportionally between a maximum voltage and a minimum voltage. The particular curve has been obtained from measured magnitudes of transmitted light in which voltages are applied at regular intervals with minimum and maximum values of positive voltages as VA and VB and minimum and maximum values of negative voltages as VC and VD.
The graph shown in FIG. 3 has a symmetric structure on the basis of voltages VB and VC. In FIG. 3, a reference mark ‘T’ represents the magnitude of light transmitting liquid crystal. Also, reference marks VA to VD represents reference voltages applied to pixel electrodes of the liquid crystal, and one graph corresponding voltages VA to VB represents transmittances of the case of applying positive voltages and the other graph corresponding to voltages VC to VD represents transmittances of the case of applying negative voltages. Herein, the values of the determined voltages VA to VD are values corresponding to the reference voltages Vref1 to Vrefn generated by the fixed reference voltage generating means, and it is very difficult to change reference voltage values after being determined.
However, liquid crystal displays have difference little by little in the slope of the voltage-transmittance graph according to manufacturing companies, so that it is required to set variable reference voltage values VA′, VB′, VC′, and VD′ in order to obtain the slope of a desired curve after maximum and minimum voltage values are determined.